Backend code for LLVM requires knowledge of how to implement the IR in native code. From there, you would have to implement the peephole optimization yourself.

The reason that the backend in my GitHub repo has languished is that it is based on a very old version of LLVM and merging it with current code would be an ordeal in itself.

I just saw there's another fork, with one commit from last September. The commit adds new code, but I'm not sure if it can be considered more complete than yours, or the opposite: https://github.com/mntmn/llvm-m68k